FIG. 5 shows a prior art multiple cell photovoltaic device shown in "Technical Digest of 2nd International Photovoltaic Science and Engineering Conference" (PVSEC-II, 1986, Peking), page 395. In FIG. 5, reference numeral 1 designates p-type polycrystalline silicon. N-type amorphous silicon 2 is disposed on the p-type polycrystalline silicon 1. P-type amorphous silicon 3 is disposed on the n-type amorphous silicon 2. Intrinsic amorphous silicon 4 is disposed on the p-type amorphous silicon 3. N-type monocrystalline silicon 5 is disposed on the intrinsic amorphous silicon 4. A transparent conductive film 6 constituting a front surface electrode is disposed on the n-type monocrystalline silicon 5. Light is incident on the device from the side of the transparent electrode 6. A rear surface Al electrode 7 is disposed on the p-type polycrystalline silicon 1. Reference numeral 11 designates a first solar cell and reference numeral 12 designates a second solar cell. These two solar cells are connected in series.
The incident light is transmitted through the transparent conductive film 6, and the short wavelength light is converted into photo-generated carriers in the intrinsic amorphous silicon 4 of the second solar cell 12 while the long wavelength light is converted into photo-generated carriers in the p-type polysilicon 1 of the first solar cell 11. These -.photo-generated carriers flow and are collected at the respective junctions, thereby generating currents. When the intrinsic amorphous silicon 4 is thick, the light absorbed by the second solar cell 12 is increased. However, the quality of the intrinsic amorphous silicon 4 is poor and there is, therefore, an upper limit to the film thickness of the intrinsic amorphous silicon 4. Therefore, even light having an energy larger than the energy band gap of the intrinsic amorphous silicon 4 of the second solar cell 12 is not completely absorbed and is transmitted to the first solar cell 11 where it generates a current.
In this way, the external current of a solar cell having a first and second solar cell serially connected with each other is restricted by the lower of the cell currents of the two cells. However, in a p-i-n cell comprising amorphous silicon, as is usually used for the second solar cell, the short-circuit current density is about 15 to 17 mA/cm.sup.2. On the other hand, the short-circuit current density of the crystalline silicon of the first solar cell is about 36 to 40 mA/cm.sup.2. A short-circuit current density of about 20 to 25 mA/cm.sup.2 is generated by the light which is transmitted to the second solar cell without being absorbed in the first solar cell. Therefore, the external current density is restricted to about 15 to 17 mA/cm.sup.2 which is the photocurrent generated in the second solar cell. The photocurrents of the serially connected cells are unbalanced, thereby reducing the photovoltaic conversion efficiency of the structure. The photovoltaic conversion efficiency in this case is about 14 to 16%.
On the other hand, as disclosed in Japanese Published patent application No. 59-96777, even if a transparent film is inserted between the first solar cell and the second solar cell, without considering the reflection characteristics of the film, balancing of the photocurrents is not effected, and the photovoltaic conversion efficiency is not enhanced.
In Japanese Published patent application No. 60-35580, an ITO (indium tin oxide) layer 1000 to 1500 Angstroms thick is inserted between the first solar cell and the second solar cell. FIG. 6 shows the reflection characteristics in this case. As is apparent from this figure, the reflected light is effectively utilized in the cell at the light incident side. In the lower cell, however, the incident light intensity is reduced to a great extent by the insertion of the ITO film.
Furthermore, as shown in Japanese Published patent application No. 63-77167, it is impossible to effectively utilize the light even when the film thickness of the ITO is 100 to 2000 Angstroms. Especially in an ITO layer 600 Angstroms thick, as disclosed in Japanese Published patent application No. 63-77167, the reflection characteristic is broad and the response to the longer wavelengths of the lower cell is significantly reduced, as shown in FIG. 7. Thus, effective utilization of light is impossible.